1. Technical Field
This invention generally relates to process for producing polyurethane foams. The invention is especially adapted for producing polyurethane foams employing the one-shot foaming process, the quasi-prepolymer process and the pre-polymer process. Specifically, the invention relates to polyurethane catalysis with a delayed action catalyst system and optionally an organotin catalyst. The delayed action catalyst is composed of at least the reaction product of (a) one or more carboxylic acids having hydroxy and/or halo functionality; (b) one or more tertiary amine ureas and, optionally, (c) one or more specific reactive tertiary amine(s) and/or one or more specific tertiary amine carbamate(s) for promoting reactions involved in the production of polyurethanes, preferably one-shot polyurethanes, and particularly flexible polyurethane foams.
2. Background
Polyurethane foams are produced by reacting a di- or polyisocyanate with compounds containing two or more active hydrogens, generally in the presence of blowing agent(s), catalysts, silicone-based surfactants and other auxiliary agents. The active hydrogen-containing compounds are typically polyols, primary and secondary polyamines, and water. Two major reactions are promoted by the catalysts among the reactants during the preparation of polyurethane foam, gelling and blowing. These reactions must proceed simultaneously and at a competitively balanced rate during the process in order to yield polyurethane foam with desired physical characteristics.
Reaction between the isocyanate and the polyol or polyamine, usually referred to as the gel reaction, leads to the formation of a polymer of high molecular weight. This reaction is predominant in foams blown exclusively with low boiling point organic compounds. The progress of this reaction increases the viscosity of the mixture and generally contributes to crosslink formation with polyfunctional polyols. The second major reaction occurs between isocyanate and water. This reaction adds to urethane polymer growth, and is important for producing carbon dioxide gas which promotes foaming. As a result, this reaction often is referred to as the blow reaction. The blow reaction is essential for avoiding or reducing the use of auxiliary blowing agents.
Both the gel and blow reactions occur in foams blown partially or totally with the in-situ formation of carbon dioxide gas. In fact, the in-situ generation of carbon dioxide by the blow reaction plays an essential part in the preparation of xe2x80x9cone-shotxe2x80x9d water-blown polyurethane foams. Water-blown polyurethane foams, particularly flexible foams, are produced by both molded and slab foam processes.
As noted above, in order to obtain good urethane foam structure, the gel and blow reactions must proceed simultaneously and at optimum balanced rates. For example, if the carbon dioxide evolution is too rapid in comparison with the gel reaction, the foam tends to collapse. Alternatively, if the gel extension reaction is too rapid in comparison with the blow reaction generating carbon dioxide, foam rise will be restricted, resulting in a high-density foam. Also, poorly balanced crosslinking reactions will adversely impact foam stability. In practice, the balancing of these two reactions is controlled by the nature of the promoters and catalysts, generally amine and/or organometallic compounds, used in the process.
Flexible and rigid foam formulations usually include e.g., a polyol, a polyisocyanate, water, optional blowing agent (low boiling organic compound or inert gas, e.g., CO2), a silicone type surfactant, and catalysts. Flexible foams are generally open-celled materials, while rigid foams usually have a high proportion of closed cells.
Historically, catalysts for producing polyurethanes have been of two general types: tertiary amines (mono and poly) and organo-tin compounds. Organometallic tin catalysts predominantly favor the gelling reaction, while amine catalysts exhibit a more varied range of blow/gel balance. Using tin catalysts in flexible foam formulations also increases the quantity of closed cells contributing to foam tightness. Tertiary amines also are effective as catalysts for the chain extension reaction and can be used in combination with the organic tin catalysts. For example, in the preparation of flexible slabstock foams, the xe2x80x9cone-shotxe2x80x9d process has been used wherein triethylenediamine is employed for promoting the water-isocyanate reaction and the cross-linking reaction, while an organic tin compound is used in synergistic combination to promote the chain extension reaction.
Flexible polyurethane foams are commercially prepared as slabstock foam or in molds. Some slabstock foam is produced by pouring the mixed reactants in large boxes (discontinuous process), while other foam is prepared in a continuous manner by deposition of the reacting mixture on a paper lined conveyor. The foam rises and cures as the conveyor advances and the foam is cut into large blocks as it exits the foam machine. Some of the uses of flexible slabstock polyurethane foams include: furniture cushions, bedding, and carpet underlay.
In the discontinuous processes, the initiation of the reaction must be delayed to allow uniform laydown of the reacting mixture and allow excess air entrapped during reactant mixing to escape. Otherwise, foam splitting caused by the tardy release of such entrapped air may occur. In such situations, delayed action catalysts can be used to achieve the required reactivity profile. The problem also can be acute with slabstock foam produced by the continuous process on a machine with a short conveyor. In this case, the formulation has to be highly catalyzed in order to be sufficiently cured when the foam reaches the cutting saw. Thus, not only is delayed action necessary for a uniform laydown, but once activated, rapid catalytic action is critical.
The process for making molded foams typically involves the mixing of the starting materials with polyurethane foam production machinery and pouring the reacting mixture, as it exits the mix-head, into a mold. The principal uses of flexible molded polyurethane foams are, e.g., automotive seats, automotive headrests and armrests and furniture cushions. Some of the uses of semi-flexible molded foams include, e.g., automotive instrument panels, energy managing foam, and sound absorbing foam.
Amine emissions from polyurethane foams have become a major topic of discussion, particularly in car interior applications, and some car manufacturers request that all Volatile Organic Compound""s (xe2x80x9cVOC""sxe2x80x9d) be reduced. One of the main components of VOC""s evaporating from flexible molded foams is the amine catalyst. To reduce such emissions, catalysts having a very low vapor pressure should be used. Alternatively, if the catalysts have reactive hydroxyl or amine groups they can be linked to the polymer network. If so, insignificant amine vapor will be detected in the fogging tests. However, the use of the reactive amines is not without difficulties. Reactive amines are known to degrade some fatigue properties such as, for example, humid aging compression set (xe2x80x9cHACSxe2x80x9d).
Modern molded flexible and semi-flexible polyurethane foam production processes have enjoyed significant growth. Processes such as those used in Just-in-Time (JIT) supply plants have increased the demand for rapid demold systems, i.e., systems in which the molding time is as short as possible. Gains in productivity and/or reduced part cost result from reduced cycle times. Rapid cure High Resilience (HR) molded flexible foam formulations typically achieve demold times of three to five minutes. This is accomplished by using one or more of the following: a higher mold temperature, more reactive intermediates (polyols and/or isocyanate), or increased quantity and/or activity of the catalysts.
High reactivity molded polyurethane systems give rise to a number of problems however. The fast initiation times require that the reacting chemicals be poured into a mold quickly. In some circumstances a rapid build-up of the viscosity of the rising foam causes a deterioration of its flow properties and can result in defects in the molded parts. Additionally, rapidly rising foam can reach the parting line of the mold cavity before the cover has had time to close resulting in collapsed areas in the foam. In such situations, delayed action catalysts can potentially be used to improve the initial system flow and allow sufficient time to close the mold. As utilized herein, the expression xe2x80x9cdelayed action catalystsxe2x80x9d shall be understood to refer to catalysts that display the desirable property of having a slow start followed by increased activity. That is, a delayed action catalyst will exhibit a low activity at first followed by increased activity at a later time. Catalysts exhibiting high catalytic activity following activation are especially useful. However, increasing the level of reactive catalysts in order to achieve good curing generally results in worsening the fatigue properties of the produced parts.
Another difficulty experienced in the production of molded foams, which is usually worse in the case of rapid cure foam formulations, is foam tightness. A high proportion of closed cells causes foam tightness at the time the molded foam part is removed from the mold. If left to cool in that state, the foam part will generally shrink irreversibly. A high proportion of open cells are required if the foam is to have the desired high resiliency. Consequently, foam cells have to be opened physically either by crushing the molded part or inserting it into a vacuum chamber. Many strategies have been proposed, both chemical and mechanical, to minimize the quantity of closed cells at demold.
The principal uses of rigid polyurethane foams are, e.g., pour-in-place insulation foams for refrigeration applications, transportation applications, and metal doors, as well as boardstock and sprayed insulation. In rigid foam applications, delayed action catalysts can also find use for the same reasons needed in flexible foam molding, to delay the initial system reactivity while offering the short cure times required for fast production cycles.
Delayed action catalysts are expected to find their main application in the manufacture of molded flexible and semi-flexible polyurethane foam parts. In such applications, it is desirable to make the molding time as short as possible (xe2x80x9crapid demoldxe2x80x9d), but the onset of the reaction must be delayed so that the viscosity increase accompanying the reaction does not jeopardize proper mold filing. Foams of a desired density can be obtained, particularly with MDI and MDI/TDI systems, from the delayed onset of viscosity build-up leading to better expansion of the reacting mixture.
Historically, delayed action catalysts used in the above-described processes are acid-blocked amines, usually simple amine salts of a tertiary amine and a carboxylic acid such as formic acid, acetic acid, or 2-ethylhexanoic acid (J. Cellular Plastics, p. 250-255, September/October, 1975). The salts are not catalytically active and, as a consequence, the amines do not activate the reaction until the salt is dissociated by the increasing temperature of the reacting mixture. Unfortunately, using carboxylic acid blocked amine catalysts generally has a tightening effect on the foam (see, e.g., U.S. Pat. Nos. 3,385,806, 4,701,474, and 4,785,027).
In the production of TDI molded foam, such as for automotive cushions, grafted polyether polyol is mixed with polyether polyol in order to obtain the desired foam hardness. Hardness often is a major limiting factor for density reduction. Because conventional delayed action, acid blocked amine catalysts (i.e., amine salts of formic, acetic, propionic and 2-ethylhexanoic acids) produce foams with lower final hardness, such catalysts are ill-suited for making lower density grade TDI molded foam.
The recent, remarkable progress made by major polyol producers to produce higher reactivity polyols, which has led to accelerated TDI molded foam curing, highlights the need for new delayed action catalysts. The high reactivity polyols tend to produce tighter foams. Since conventional delayed action acid-blocked amine catalysts also give tight foams, their conjoint use with the newer polyols exacerbates the tightness problem. Indeed, it becomes difficult to crush the foam without destroying the foam structure.
However, the need remains in the polyurethane industry for additional catalysts having a long initiation time. Most importantly, these catalysts should delay the onset of the isocyanate-polyol reaction, exhibit good curing rate, and provide excellent physical properties of produced parts. In addition, these catalyst should be capable of being incorporated into the polymer structure (i.e., reactive catalysts).
3. Description of Related Art
The use of acid-grafted polyether polyols as reactivity controllers for the production of polyurethane foams is disclosed in U.S. Pat. No. 4,701,474. Such acid-grafted polyether polyols purportedly reduce the reactivity of polyurethane foam formulations without the tightening effect which usually results from using carboxylic acid-amine salts. The number average molecular weight range claimed for the disclosed acid-grafted polyether polyols is 1,000 to 10,000.
Preparing polyurethane foams in the presence of polyether acids is disclosed in U.S. Pat. No. 4,785,027. The polyether acids are mono- or di-acids with the acid functional groups located at the ends of the polymer chains. The polyether chain is built from ethylene and/or propylene oxide to have repeating oxyalkylene groups. In the case of mono acids, the other terminal group can be an alkyl or hydroxyl function. The presence of the hydroxyl functional group is optional. Such polyether acids purportedly delay the initial reaction rate without increasing foam tightness observed with formic acid-amine salts. It is stated that the system has an advantage over systems based on formic acid in that the polyurethane foam is not tight and does not suffer from skin peeling.
In U.S. Pat. No. 4,366,084, the fuming of dimethylaminopropylamine (DMAPA) is reduced by blocking the amine with phenol. The reduction in fuming increases directly with the percent blocking. According to the patent, using the DMAPA-phenol salts at varied blocking ratios does not cause any deterioration in the air flow and compression set properties of the foam.
U.S. Pat. No. 5,179,131 discloses that the addition of mono- or dicarboxylic acids to polyurethane foam formulations made using polyisocyanate polyaddition polymer poly-dispersions results in a reduction in foam shrinkage. The functional groups attached to the acid are either alkyl or alkylene.
The use of the amine salts of tertiary amino-acids as delayed action catalysts in the production of polyurethanes is disclosed in U.S. Pat. No. 4,232,152.
The use of particular N-hydroxyalkyl quaternary ammonium carboxylate salts as delayed action catalysts for the production of polyurethane is disclosed in U.S. Pat. Nos. 4,040,992 and 4,582,861 and EP Patent No. 0 484 749.
The use of particular aliphatic tertiary monoamines, and the carboxylic acid salts thereof as catalysts, in the production of polyurethane foam is disclosed in U.S. Pat. Nos. 4,450,246 and 4,617,286; U.K. Patent No. 879,167 and in Canadian Patent No. 651,638. A variety of organic mono or dicarboxylic acids are disclosed. Canadian Pat. 651,638, describes preparing polyurethane foams from an isocyanate-terminated polytetramethyleneether or polypropyleneether polyurethane prepolymer and water, in the presence of an acid-amine salt. In certain examples, salts of the hydroxy-acid, citric acid and either N-methyl morpholine or triethylamine are specifically exemplified. U.K. Patent 879,167 describes using a tertiary amine salt of lactic acid.
U.S. Pat. No. 2,932,621 discloses the use of dimethylethanolamine salts of dicarboxylic acids (such as oxalic acid) as a catalyst in the preparation of polyurethane foam.
U.S. Pat. No. 3,728,291 describes the use of triethylenediamine (TEDA) salts of formic acid in combination with 1-(2-hydroxypropyl) imidazole as a catalyst to permit wider tin latitude in the preparation of polyurethane foams.
U.S. Pat. Nos. 3,862,150 and 4,165,412 discloses the use of tertiary amines and substituted carboxylic acids as catalysts in preparing polyurethane foams. The acid must have a carboxyl group at one end of the molecule and a group selected from CN, SO, SO.sub.2, CO, NO.sub.2, COCH.sub.3 and CO-phenyl on the other end. An example is the salt of TEDA and cyanoacetic acid.
European Patent No. 0,088,377 discloses a method for producing carbamate and carbonate salts of tertiary amines which comprises admixing secondary and tertiary amines in the presence of carbon dioxide.
European Patent No. 0,361,937 discloses the use of carboxylic acid salts to slow down the reaction between aminated polyether polyols and isocyanate, so that effective foaming can take place before the reaction is completed.
European Patent No. 0,140,480 discloses the use of monocarboxylic acid salts of bis-(aminoethyl) ether derivatives as catalysts for the preparation of polyurethane foams.
More recently, it was discovered that salts of a tertiary amine and a carboxylic acid with hydroxyl and/or halo functionality could advantageously be used as delayed action catalysts for promoting reactions involved in the production polyurethanes, including one-shot polyurethanes, and particularly flexible polyurethane foams. This technology is described in U.S. Pat. Nos. 5,489,618, and 6,395,796 and E.P. Patent No. 0 656 383. The use of such amine salts results in the manufacture of polyurethane foams which are either more open or more easily opened, or both. Also, U.S. Pat. No. 6,387,972 describes the use of specific reactive tertiary amine salts to improve humid aging compression set
It is an object of the present invention to provide a delayed action catalyst system to catalyze the reaction between an isocyanate functionality and an active hydrogen-containing compound, e.g., an alcohol, a polyol, an amine, water, etc., to make polyurethane foams.
Accordingly, a process for preparing a polyurethane foam is provided which comprises reacting a polyisocyanate and an active hydrogen-containing component, including water and an organic polyol, in the presence of a catalytically effective amount of a delayed action amine catalyst system comprising a reaction product of (a) one or more carboxylic acids having hydroxy and/or halo functionality; (b) one or more tertiary amine ureas; and, optionally, (c) a reactant selected from the group consisting of specific reactive tertiary amine(s), specific tertiary amine carbamate(s) and mixtures thereof. If desired, one or more organotin catalysts can be employed in the polyurethane process.
Further in accordance with the present invention, a polyurethane foam is provided, the polyurethane foam having repeating units derived from the reaction of an organic polyisocyanate and an active hydrogen-containing component, including water and an organic polyol, and a catalytically effective amount of a delayed action amine catalyst system comprising a reaction product of (a) one or more carboxylic acids having hydroxy and/or halo functionality; (b) one or more tertiary amine ureas; and, optionally, (c) a reactant selected from the group consisting of specific reactive tertiary amine(s), specific tertiary amine carbamate(s) and mixtures thereof. If desired, one or more organotin catalysts can also be employed.